Theodore Sturgeon once noted that Kirk as a concept wasn't distinctive, so it absolutely fell to Shatner to bring more to it ... and then hope the writers picked up on the strengths. Clearly Gene Coon knew how to do it, giving him the speeches (which manage sometimes to be cringe-worthy and powerful at the same time, don't ask me how.)
Shatner's own notion of wearing command like an old comfortable jacket is the thing that absolutely works for me; I find that a much more progressive view of command than the king-on-throne aspect that Stewart often offers up on screen, and that is probably why I like Brooks (when he isn't asleep) too. It's why I would have liked Janeway if VOYAGER hadn't been unwatchable, too.
Having said all that, I still think the quiet-pro Kirk of pre-Coon TREK to be at times awesome; 'go to your quarters or I'll pick you up and carry you there' is just pitch-perfect. The relative mildness of Kirk in the movies seems less like affectation than just sounding the wrong note ... kind of like Sean Connery's sudden verging on Roger Mooredness in NEVER SAY NEVER AGAIN, where Sean's 'eye of the tiger' appears during the opening 'kobayashi maru' ripoff but never during the film proper.